Experience with oil and natural-gas exploration, production and transmission in the North Sea has shown that, because of the frequent bad weather, surface vessels can be deployed in such work only on a limited scale. The time spent coming and going from and to the nearest port to avoid storms represents a major loss of working time for surface vessels engaged in the North Sea project.
Underwater craft suited for undersea work usually are relatively small and can be employed only in conjunction with a mother ship on the surface. When the mother ship has to leave the area because of impending bad weather, it is forced to take the dependent submersible craft along, even though the submersible craft would in principal avoid the heavy sea conditions by submerging below a relatively shallow depth.
Submersible craft with a surface buoy serving as power-supply unit are similarly affected by bad weather.
German Offenlegungsschrift 23 56 537 of the present inventor discloses a catamaran surface vessel having a submersible gondola located between the two hulls of the catamaran. The catamaran remains afloat when the gondola is lowered for underwater travel. The submersible gondola has a torpedo-like shape widely used for self-propelled underwater craft because of its low resistance to motion for the volume of water displaced, relative to other shapes. Although the catamaran, by virtue of the form stability provided by its two spaced-apart hulls, has a high resistance to capsizing, it is nonetheless susceptible to bad weather and rough seas as are other surface vessels.
U.S. Pat. No. 1,757,174 to Douglas discloses a seagoing vessel having five pontoons: a cabin pontoon, two waterline pontoons, and two power pontoons. The two waterline pontoons are disposed below and to either side of the cabin pontoon. A power pontoon is disposed below each waterline pontoon. The vessel of U.S. Pat. No. 1,757,174 is a surface vessel. Only the power pontoons, located beneath the waterline pontoons, are submerged when the vessel is under way. Consequently, the vessel is also affected by heavy seas.